Photoconductive semiconductor switches (PCSS) can bemused for the switching of high-speed, high-power, high-voltage electronics and optoelectronics. These uses include pulsed power applications as diverse as low-impedance, high-current firing sets in munitions; high-impedance, low-current Pockels cell or Q-switch drivers for laser diode arrays; high-voltage, high-current compact accelerators; and pulsers for ground penetrating radar. PCSS have demonstrated significant improvement over conventional pulsed power switching technologies, including 100 ps rise time, kilohertz (continuous) and megahertz (burst) repetition rates, scalable or stackable to hundreds of kilovolts and tens of kiloamps, optical control and isolation, and inherent solid-state reliability. See Loubriel at al., “Photoconductive Semiconductor Switches,” IEEE Trans. Plasma Science 25(2), 124 (1997), and U.S. Pat. No. 5,804,815 to Loubriel et al., which are incorporated herein by reference.
In particular, optically triggered PCSS can provide high-voltage isolation due to their being optically, rather than electrically, initiated. Furthermore, PCSS can be made radiation-hardened by reducing the carrier lifetime of semiconductor material in the switch gap, such as by neutron irradiation. PCSS are therefore useful in radiation environments and in environments that have electrical interference. For example, optically triggered PCSS can be made immune to lightning strikes that would accidentally trigger electrically activated devices.
With an optically triggered PCSS, the energy of the incident photons excites electrons from the valence band to the conduction band of the semiconductor. Conventional PCSS produce only one electron-hole pair per absorbed photon. In this linear-mode, the excitation is independent of the electric field across the switch. Therefore, the conventional PCSS can be operated to arbitrarily low voltage. For example, a GaAs PCSS operates in the linear mode at electric fields below about 4 kV/cm. However, this linear mode operation requires a high power laser to optically trigger the PCSS and achieve high current switching. Furthermore, after the exciting laser light is extinguished, the carrier density only slowly exponentially decays, in 1–10 ns.
At higher electric fields, these switches can behave very differently. At high electric fields, a light source can trigger photo-excited carriers which then can collectively impact ionize additional carriers, resulting in avalanche carrier generation. Thus, one photon can produce many current carriers in a high-gain PCSS. Because the high electric field induces carrier multiplication, the amount of light required to achieve high current switching is reduced by as much as five orders of magnitude compared to the linear mode PCSS. For example, a 100 kV GaAs PCSS can be triggered with less than one microjoule of optical energy. Thus, extremely low energy light pulses, such as are available with a small laser diode array, can be used for optical triggering.
Another aspect of high-gain PCSS is a voltage drop during conduction. Once triggering is initiated, the high-gain PCSS continues to generate carriers until the field across the switch drops to a “lock-on” field (about 4–6 kV/cm in GaAs). In the “on” state the field across the high-gain PCSS stabilizes to this constant lock-on field. The switch current is then circuit-limited, and the switch will conduct whatever current is necessary to maintain the constant lock-on voltage until the energy in the circuit is dissipated.
However, in high-gain PCSS, the current flows in filaments. During high-gain switching, the PCSS emits bandgap radiation due to carrier recombination, which can be detected optically with a near-infrared sensitive camera. When this radiation is imaged, filaments are observed, even if the triggering light is uniform. The filaments can have densities of several megamperes per centimeter squared and diameters of 15–300 μm.
The lifetime of the PCSS is determined by circuit parameters, trigger properties, switch properties, and, in particular, the ability of the contact electrodes to resist erosion due to current filamentation. The high current density in a filament causes localized heating and damage at the contact boundary where the current enters or exits the semiconductor in the gap region of the switch. Subsequently, metal contact erosion causes degradation of switching conditions and eventual failure of the switch function. Furthermore, switch lifetime drops dramatically as the current is increased. Therefore, existing high gain PCSS are limited to applications wherein the switch is to be used for a limited number of low-current pulses. What is needed is an improved PCSS with a longer lifetime (longevity) for applications requiring higher current and longer duration switched pulses.
The present invention provides multiple-line triggering and an interdigitated electrode structure for an improved PCSS. The invention increases the switch lifetime at high currents by controlling the formation and number of multiple simultaneously generated parallel filaments to share the current, reducing the peak current density and damage. Furthermore, the present invention can mitigate the degradation of switching conditions with increased number of firings of the switch.